Mikuni TM-40 Flatslide Jetting Thread

[Gotlabs]

Alright, I got my junk streetable again. Checked the float levels, changed the main to 155.

still has a slight bog, but way less than I've been battling the past week. Just waiting a #40 ap nozzle to get delivered and that should eliminate the rest of the bog.

[MotorGus]

Need help here:

Used TM40

Main jet : 145

Pilot : 32.5

Needle : Y4, clip on 3rd groove from top

Pilot Air : 1.1

Bike

97 DR650SE

larger header

tunning camshaft

uncut, de-snorkeled airbox

GSXR can

Pilot screw adjusted: 2 turns out

Symtoms:

A bit hard to start.

Idles fine, engine response is crisp, with a noticeable bog when opening the throttle, after that it will recover and may even pull the front wheel. Bike feels unpredictable, not fun.

delayed the AP operation to 1/5 throttle and improved driveability, but still not close to be acceptable

Plugs out: black sooted, very rich mixture.

It seems like a setup for a full cut airbox.

Your opinions please

[ProCycle]

The pilot is way too big. Should be 22.5

It also sounds like it the accelerator pump nozzle is way too large.

[MotorGus]

Given my by bike current setup, what would be the best carb setup?

I've been suggested to use 160 main, 40 pilot, needle y4, clip on 3rd groove and one turn out on the fuel screw.

I'm also unfamiliar with the nozzle, is the golden thingy that sprays fuel into the slide?

[thumpididump]

Anyone know where I can get a replacement rubber seal for the bowl on my new TM-40? I've had the bowl off twice since I bought it a few weeks ago and both times I noticed the rubber gasket didn't fit very well at all... seems too big and had a real hard time to get it on there. I don't want to remove the bowl again until I can have a replacement seal on hand.

[Bob Onit]

Anyone know where I can get a replacement rubber seal for the bowl on my new TM-40?

Jeff at Pro-Cycle?

A very light film of grease will hold it in place during assembly as well.

I have a quick question about your TM40 tuning (from your web site) if you don't mind.

Did your bike lift the front wheel any easier with the "throttle only" after you switched back to the stock pilot and lowered the needle one slot?

I'm having similar a problem with a new TM33 on a DR250. It's either too rich or too lean down at low RPM. It acts like a CV until about 1/4-1/3 throttle. Wont lift the front wheel and its not nearly as responsive as my other TM33 which is on my DR350

Thanks.

Edited March 30, 2010 by Bob Onit

[thumpididump]

Mainly, what I noticed after dropping back to the #22.5 pilot jet was that the throttle seems snappier. Mostly, when you give it a really quick snap at idle, it there is no bog. With the #20, there seemed to be a slight bog if your hand was really quick.

It's still not perfect, but better than when compared to the few days I rode with the #20 pilot jet. So, yes, the front wheel seems able to lift off a little easier with a quick snap of the throttle.

Just for kicks, I might attempt going back to the #20, putting the needle back to the 3rd clip position, and adjusting the accelerator pump to open a little earlier. I haven't touched the AP adjustment yet. It's all about trial and error. I basically live at sea level and my airbox is opened up à la Jesse Kientz but I've cut my hole a little larger than the one shown in his photo, and I'm running a freshly oiled K&N. I haven't touched the main jet because it runs just fine from 1/2 to WOT (or at least I want to make sure I get the lower throttle running perfect before I go messing with the main jet).

The spark plugs look pretty black & sooty around the outer area, but the centre of the plug (area around the electrode) looks tan and maybe even a little white. Sticking my finger into the outlet of the muffler, it's dirty and sooty. One of these days I'll nail the correct balance.

[Bob Onit]

It's all about trial and error.

Yes it is ?

Unfortunately I'm having too much error for the trial I'm putting in to it.?

Thanks for the reply.... I'll keep plugging away at it.

By the way, you could probably pick up a new bowl gasket from a Harley dealer if you have one local ?

Thanks again

[thumpididump]

Never thought about that.... Does anyone have an idea which model/year HD came with this carb?

,...

By the way, you could probably pick up a new bowl gasket from a Harley dealer if you have one local ?

Thanks again

[Bob Onit]

Never thought about that.... Does anyone have an idea which model/year HD came with this carb?

Post #6

https://www.thumpertalk.com/forum/showpost.php?p=6797247&postcount=6

I believe the 1340cc was the first of the EVO engine which appeared in about 1984-85 or so.

I would think that any Harley dealer will be able to tell you what year/model the TM40's came on. ?

[ProCycle]

Never thought about that.... Does anyone have an idea which model/year HD came with this carb?

None. Just like on the DR650, the TM40 was a good way to replace the lame stock carburetor. It was a very popular performance improvement for Sportsters.

[Bob Onit]

None.

Hmm. So do you suppose the TM40 was built for upgrade only and was never actually used on a production bike? ?

[ProCycle]

Hmm. So do you suppose the TM40 was built for upgrade only and was never actually used on a production bike? ?

I don't know the original history of the TM40 but back in the dim past when I worked the parts counter at a Harley dealer we sold a ton of TM40 (HS40) performance kits.

Mikuni makes several of their own HD carb upgrade kits.

--> http://www.mikuni.com/fs-products_guides.html

HD - just like everybody else - ran CV carbs when emissions requirements were put in place. Prior to that they used Bendix/Zenith carbs.

[MotorGus]

Given my by bike current setup, what would be the best carb setup?

I've been suggested to use 160 main, 40 pilot, needle y4, clip on 3rd groove and one turn out on the fuel screw.

I'm also unfamiliar with the nozzle, is the golden thingy that sprays fuel into the slide?

Forgot to mention this setup was recommendeed for a top cut open airbox.

What do you think?

[ProCycle]

Forgot to mention this setup was recommendeed for a top cut open airbox.

What do you think?

Way too rich.

22.5 pilot

150 main

Yes the accelerator pump nozzle squirts fuel every time you open the throttle.

[MotorGus]

Got my TM40 working on the DR650 with jets: Main 145 and Pilot 32.5, needle clip on the 2nd groove from top, and top cut air box.

Runs nice, but could be nicer. Plugs have the brown tan color, no black smoke, MPG seems good too.

Low end has lots of grunt, wheelies are coming naturally with 16-43 tranny on 1st and 2nd gears, however it still seems medium to top end is a bit underpowered, and sometimes when sitting idle at the traffic lights, occasionally it literally misses a beat, for less than a second i feel the engine stalling and then getting back to idle. Maybe i need to play a bit more with the pilot screw!?

And finally, i keep forgetting to ask this: there are two hose exits, one on each side, are these venting channels, each has an individual hose, or these are to be connected with one hose?

[BSR-1]

you guys that are having trouble with pilot sizing, check here:

https://www.thumpertalk.com/forum/showthread.php?t=327405

[thumpididump]

32.5 pilot sounds waaayy too big considering ProCycle's carb setup works pretty good as shipped with a 22.5 ..I'd guess the stumbling at idle is because it's dumping too much gas in there, and adjusting the screw would have to screw it all the way in to compensate.

if mid-top end feels weak, then try dropping the needle to the 3rd clip position from the top. This will raise the needle in the main jet when the throttle is opened up in that mid-range.

You're running way too rich in low rpm and too lean in upper rpm.

edit: a good read in the tuning section of this manual:

http://www.mikuni.com/pdf/hs40_manual.pdf

Got my TM40 working on the DR650 with jets: Main 145 and Pilot 32.5, needle clip on the 2nd groove from top, and top cut air box.

Runs nice, but could be nicer. Plugs have the brown tan color, no black smoke, MPG seems good too.

Low end has lots of grunt, wheelies are coming naturally with 16-43 tranny on 1st and 2nd gears, however it still seems medium to top end is a bit underpowered, and sometimes when sitting idle at the traffic lights, occasionally it literally misses a beat, for less than a second i feel the engine stalling and then getting back to idle. Maybe i need to play a bit more with the pilot screw!?

And finally, i keep forgetting to ask this: there are two hose exits, one on each side, are these venting channels, each has an individual hose, or these are to be connected with one hose?

[MotorGus]

Thumpididump, will order a few jets so i can mess around.

regarding this: there are two hose exits, one on each side, are these venting channels, each has an individual hose, or these are to be connected with one hose?

could not find any info regarding it...

[thumpididump]

The one on the right side is a vacuum... if you're using the OEM vacuum-controlled petcock, then you should have a hose from there to the petcock. If you're using some type of aftermarket petcock then you can just plug off the vacuum port on the carb. On the left side, you should only have the fuel supply hose. All the others are smaller diameter hoses for venting and/or overflow. These should have long thin hoses going down to the bottom of the engine/swingarm area for draining.